The invention relates to electromagnetic energy radiating elements of the type usable in an array antenna, and more particularly to a method for maintaining an effective electrical pathlength of about one-quarter wavelength between the radiator and groundplane over a multi-octave band of frequencies.
Certain radiating elements, for example, a slot in a groundplane, radiate with equal amplitude in both the forward and back directions. In order to utilize this type of radiator in a practical array antenna, the back-directed wave must be taken into account. The most common approaches are either to absorb the misdirected signal in an RF load material or to recapture it by means of a reflecting groundplane spaced the proper distance behind the radiator. When this spacing is nominally one-quarter wavelength at the operating frequency, the forward and reflected waves reinforce one another to produce maximum radiation efficiency.
The drawback with suppressing the back-directed wave with absorbing material is that one-half of the total energy is lost in the absorber. Nevertheless, ultrawide-band performance is achieved, which makes this type radiator attractive for passive surveillance systems. Active systems, however, normally cannot tolerate the excess roundtrip loss of 6 dB as transmit power would need to be quadrupled in order to keep the same range performance. Absorber loading is described in R. C. Johnson and H. Jasik, "Antenna Engineering Handbook." New York: McGraw-Hill, 1984, pages 14-14 through 14-24.
Radiator efficiency can be maximized by means of a properly spaced, reflecting groundplane or alternately, a cavity of the proper depth. FIG. 1 shows how radiator gain varies with cavity depth in wavelengths. Equivalently, gain falls off 3 dB at 0.5 and 1.5 times the band center frequency. Operation outside this band leads to phasing problems with the two signals, i.e., further reduction in gain and eventually, pattern nulls from destructive cancellation. Furthermore, the radiator cannot be arrayed in tight lattices as the cavity must be made large enough to remain above cut-off at the lowest operating frequency.
The reflecting groundplane behaves similarly to a cavity. FIG. 2 illustrates how gain (equivalent here to radiation efficiency) varies with space, S, between the dipole radiator and the groundplane. When the effective electrical path length between the radiator and groundplane is one-quarter wavelength, the reflected energy will be in phase with the directly radiated energy, thereby reinforcing the directly radiated energy to produce the maximum radiator efficiency. The signals are in phase because there is a 90.degree. lag due to travel to the reflecting surface, a 180.degree. phase reversal resulting from the reflection, and another 90.degree. lag due to travel back to the radiator, thus totalling 360.degree.. Note that at a spacing of one-half wave-length, the gain drops to zero due to cancellation of the forward and reflected waves. The use of a reflecting groundplane is described in J. D. Kraus, "Antennas," New York: McGraw-Hill, 1950, at page 327.
It is therefore an object of the present invention to provide a radiating element comprising a radiator and a reflecting groundplane located at an effective electrical pathlength from the radiator which is maintained at about one-quarter wavelength over a multioctave band of frequencies.
A further object is to provide a dielectric material having a dielectric constant that varies in some inverse manner with frequency, preferably as 1/f.sup.2.